1. Technical Field
The invention relates to ringfield projection pattern delineation expediently used in the fabrication of devices, e.g. integrated circuits, using submicron design rules, as well as to apparatus of such design as to serve in such fabrication. Utilization of reflective optics facilitates operation within the x-ray spectrum--in particular, at "long" wavelengths at and below 300 .ANG. down to tens of .ANG.. A variety of design features are of particular consequence. Throughput capability in such fabrication results from lens design permitting substantial width as well as substantial straight line length of the scanning ringfield slit--factors resulting also in the ability to pattern delineate otherwise desired large dimensioned LSI chips within a single scan sequence. Fabrication of Very Large Scale Integrated devices--electronic as well as optical and hybrid--built to design rules of 0.25 .mu.m and below is contemplated.
2. Description of the Prior Art
The saga of Large Scale Integration from inception to the present time is well known. Evolution of the present 1-2 megabit chip built to design rules at or slightly below 1 .mu.m does not represent the ultimate product. Lithographic definition has played, and will continue to play, a significant role. Fabrication of state-of-the-art devices depend on use of near-ultraviolet wavelength (e.g. of .lambda.=3650 .ANG.). Intensive development effort is based on next generation devices which use shorter wavelength--or "deep UV" (e.g. of .lambda.=2480 .ANG.). Still smaller design rules will likely dictate use of shorter wavelengths, and it is expected that electromagnetic radiation used for patterning will necessarily be in the x-ray spectrum.
R&D activity concerned with x-ray fabrication of future generation devices has concentrated on x-ray wavelengths of .lambda.=10-15 .ANG.. Considerations relating to characteristics of materials--poor reflectance, transmission/absorption--are all considered to preclude useful optics. In consequence, this x-ray effort has been largely directed to "proximity printing"--effectively contact printing with small separation to protect fragile masks--an approach necessarily limited to 1:1 object-to-image size ratio.
Despite the above problems, a number of considerations support a continuing parallel effort directed to projection X-ray lithography. The effort is largely due to hopes for object-to-image reduction. Fabricating of 1:1 masks, particularly at or below 0.25 .mu.m design rules, presents problems while enlarged masks of high integrity, suitable for use in reduction systems, may be made using present technology. In addition, thick, robust reflective masks, precluded in proximity printing, may be used in projection.
Early efforts on projection x-ray lithography used the two spherical mirror Schwarzschild system. (See, "Soft x-ray Reduction Lithography Using Multilayer Mirrors" by Kinoshita, et al, J. Vac. Sci. Technol., vol. B6, p. 1648 (1989) and "Soft X-ray Projection Lithography Using an X-ray Reduction Camera" by Hawryluk and Seppala, J. Vac. Sci. Technol., vol. B6, p. 2161 (1988).) There are three primary factors which restrict the use of a Schwarzschild design for practical lithography-field curvature, difficulty in achieving telecentricity, and degradation due to central obscuration.
Promising work concerns use of ringfield scanning systems. All ringfield optical forms are based on radial dependence of aberration and use the technique of balancing low order aberrations (i.e. third order) with higher order aberrations to create narrow annular regions of correction away from the optical axis of the system (regions of constant radius, rotationally symmetric with respect to the axis). The shape of the corrected region is, in consequence, an arcuate strip rather than a straight strip. Strip width is a function of the smallest feature to be printed with increasing residual astigmatism at distances greater or smaller than the design radius being of greater consequence for greater resolution.
A relevant apparatus based on this principle, but for use at longer wavelengths, is the Offner 1:1 ringfield system marketed by Perkin-Elmer Corporation. Useful description is included in the disclosure of U.S. Pat. No. 3,748,015, issued Jul. 24, 1973. It is a zero distortion, telecentric system used at unit magnification. Because of the symmetry of this system (here we refer to the fact that the optical system and beam paths are effectively identical from the object to the stop as they are from the stop to the image--the "stop" refers to the point of crossover of principal rays, or the position in the system at which the aperture is generally placed) coma and distortion are intrinsically corrected. Balancing of low and high order astigmatism provides a narrow circular region of correction. A significant advantage of the Offner 1:1 system is that the ringfield correction is achieved with spherical mirrors. Disadvantages of this system are that it has no reduction, and throughput is limited by the small ring width corresponding with the narrowness of the corrected zone.
As noted, a primary motivation toward x-ray projection lithography is the difficulty in 1:1 mask fabrication as required both in proximity printing and in the Offner ringfield system. Relevant disclosures describing systems using all-reflective three element objectives, are included in U.S. Pat. No. 4,240,707, issued Dec. 23, 1980, and U.S. Pat. No. 4,733,955, issued Mar. 29, 1988. Both these designs are based on configurations which comprise a negative (concave) mirror interposed between two positive (convex) ones, and both are designed for the visible/IR part of the spectrum. These systems are well suited for imaging distant objects (at infinity) over either two dimensional or high aspect ratio (straight slits) fields of view. However, neither of these systems is telecentric in the image space nor is corrected for image distortion. Another disadvantage of three-mirror designs for the purpose at hand is that both object and image are located on the same side of the system. The result is severe restriction in wafer motion to avoid obstruction of the optical system.
Another relevant disclosure is included in U.S. Pat. No. 4,747,678 issued May 31, 1988. It describes a 4:1 reduction optical system for deep UV lithography, which uses a combination of a four mirror reflective system and three groups of refractive lens systems. This is a ringfield system; it is telecentric and has low distortion. The fundamental concept in this design is that the refractive lenses allow for use of spherical mirrors in reduction configuration. This system achieves 0.5 .mu.m resolution at 2500 .ANG. wavelength, but is not suitable for the soft x-ray wavelength region due to significant absorption by the refractive lenses, leading, in turn, to concentration on reflective lenses.
Despite the inherent advantages of the 1:1 ringfield approach as used in the scanning mode, there is little confidence that it will play a valuable role in LSI fabrication. Aside from the 1:1 mask fabrication problem, the general belief is that throughput of such a scanning system is inherently small due to extreme demands placed on minimization of aberrations across the width of the ringfield. Consequent narrow ring-shape slit results in very low throughput.
In a reduction form of a ringfield system, coma and distortion are no longer cancelled by the symmetry of the configuration. As a result, more design variables are required for the control of these aberrations in addition to spherical aberration, astigmatism and field curvature. In the design described in U.S. Pat. No. 4,747,678, the variables take form of additional refractive lenses. For the x-ray designs the additional variables are in the form of aspheric departures of each mirror from a sphere. Aspheric coefficients for third order aberration correction (often called conic constants) and for higher order correction are very effective means for controlling aberrations and, generally, each coefficient can control or correct one order of one aberration term. The use of aspheric mirrors is currently thought to be inevitable for projection x-ray system. All-spherical mirror designs, promising from the standpoint of aberration correction, have been developed, but these systems involve up to ten mirrors. Poor reflectivities of soft x-rays at near-normal incidence--.about.60% for available multi-layer mirrors--is well below the reflectivity of optical/UV wavelengths (typically 98% and higher) and this imposes a severe restriction on a number of mirrors which can be used in the projection system.
Other difficulties are perhaps of still greater consequence. The 10-15 .ANG. wavelength x-ray radiation used in proximity printing is outside the range of presently available x-ray optics as discussed above.
This has led to consideration of longer wavelength radiation-in the range centering about 130 .ANG., e.g. in the range of 100-150 .ANG.. While suitable materials of somewhat larger refractive index values at such longer wavelengths are available, limitation on throughput based on slit width, W--a limitation related, not to radiation wavelength, but to device design rules continues to be a problem. (See, Wood, et al, Proc. of 33rd Int. Symp. on Electron, Ion, and Photon Beams, "Short Wavelength Annular-field Optical Systems for Imaging Tenth Micron Features", (1989).)
Available refractive indices at .about.100 .ANG., while still quite low, are sufficient to permit fabrication of multi-layered reflective optics ("Distributed Bragg Reflectors"). DBR reflective optics resulting in .about.45% reflectance for use with 140 .ANG. radiation have been constructed and used to obtain one-tenth .mu.m feature sizes. See, Bjorkholm et al, "Reduction Imaging at 14 nm Using Multilayer Coated Optics: Printing of Features Smaller than 0.1 .mu.m", J. Vac. Sci. Technol., B8 (6) (November/December 1990). This approach, providing for full feature (non-scanning), reduction projection is severely limited by field curvature. While needed resolution is obtainable, field size is very small, e.g. 25 .mu.m.times.50 .mu.m for feature size of 0.1 .mu.m. While adaptable for use in ring-field scanning, the same field limit applies to result in a slit width W, of the order of a few microns (corresponding with a slit length, L, of the desired several mm).
In short, the lure of ring-field projection printing e.g. for 0.25 .mu.m and below due to considerations such as mask safety and possibility of object-to-image reduction, has not yielded to feasible processing. Very high camera/source costs, due to throughput limitations, are considered responsible.